Hitherto, as curing agents that exhibit a low-temperature fast-curing activity with respect to epoxy resins, there have been proposed aluminum chelate-based latent curing agents in which porous resins obtained by interfacial polymerization of multifunctional isocyanate compounds support aluminum chelate-based curing agents (for example, see PTLs 1 and 2).
When aryl silanol is used in combination in thermosetting epoxy resin composition using such latent curing agents, the aluminum chelate and aryl silanol work in cooperation with each other to allow the epoxy resin to be cationically cured.
With a view to, for example, imparting weather resistance to thermosetting epoxy resin compositions or forming relatively thickly coatable coating films of thermosetting epoxy resin compositions, there has been proposed a technique of blending an aluminum chelate, alkyl phenyl polysiloxane, and an alkoxy boron compound (for example, see PTL 3).
In recent years, thermosetting epoxy resin compositions using latent curing agents have been required to have a greater low-temperature curability. However, an attempt to improve low-temperature curability typically causes a problem of increasing the likelihood of viscosity rise during storage.